The requirement that a status indicator should have a reliable long-term memory in principle excludes the use of lamps and light-emitting diodes for this purpose. The need for a memory function suggests that the indicator be substantially formed as a relay with a flag or as an electromagnet with an iron core and a magnetizing winding and with some form of permanently-magnetized indicating disk which is movable magnetically between its bistable rest states (e.g. rotatable through 180.degree.). This invention relates to a status indicator according to the latter alternative, that is, where indication is performed with a rotatable disk which, when one of its surfaces faces in the viewing direction, indicates a normal state and which, after an activation pulse, turns its other surface to the viewing direction. to be able to distinguish the two states from each other in a simple manner, widely contrasting colours can be used on the two surfaces.
When the electromagnet is not magnetized by the passage of current through the energising winding, there will still be some residual magnetism in the electromagnet due to the remanence. The north-magnetic pole of the permanent magnet located on the rotating disk will then be attracted towards the south-magnetic pole (determined by the remanence), of the electromagnet and vice versa.
From a non-indicating or normal state, the indicator is activated and indicates a change of state in the monitored object when the electromagnet is magnetized with the aid of a current pulse in the energising winding. The winding and current directions are then assumed to be such that the magnetic poles of the electromagnet change polarity in relation to the existing remanence. The disk is thereby rotated through 180.degree. and presents to the viewing direction that surface which indicates that a change of state has taken place. Owing to the remanence existing in the electromagnet, even after the indicating current pulse has died away, the disk will remain in the indicating position, so that a reliable memory function is obtained.
There are a number of manufacturers of status indicators who utilize the principle described above, for example Ferranti-Packard (Canada) and Sasse (Fed. Rep. of Germany).
Resetting of the indicator to a non-activated condition is performed either by conducting a current pulse of opposite polarity through the energising winding, whereby the polarity of the magnetic poles of the electromagnet is changed, or by conducting a current pulse through a special reset winding.
However, the above resetting method presupposes that a source of current be available, which is not the case in several applications, for example where a d.c. battery is missing. If, for example, the indication is of a fault on the power supply system, the part subjected to the fault is normally disconnected. In such cases it may be desirable to reset the indicator before any fault-removing measures are taken for example to see whether the fault has disappeared after a reclosing, to be able to see any consequential faults, and so on.
The problem of resetting the indicator when no current is available for resetting has existed for a long time. For individual indicators this has been solved in such a way that a relatively strong permanent magnet has been built into a casing comprising the indicators and a manual operating member for the permanent magnet. By moving the permanent magnet--the poles of which are oriented with opposite polarity in relation to the remanent magnetic polarity existing in the electromagnet after the latter has been activated by a current pulse--past the electromagnet, the latter is re-magnetized in the direction to again move the indicating disk through 180.degree. and return the indicator to its non-activated state.
However, manual permanent-magnetic reset devices available on the market have certain limitations and cause problems in several respects:
They exist in a form which is integrated with the status indicator, i.e., each device has its own remagnetization unit.
Existing status indicators with a manual magnetic reset facility cannot be mounted adjacent each other in any orientation since the permanent magnet for resetting which is included has such high magnetic energy that its field may disadvantageously affect an adjacently positioned status indicator.
These integrated design solutions are considerably more space-demanding than the status indicator alone.
These facts taken together mean that the necessary space for a number of status indicators with magnetic reset facility will be considerably greater than for the same number of indicators without a reset facility.
The requirement for compact integrated system solutions where status indicators with manual magnetic resetting are included is thus impossible to fulfil.
From the point of view of cost, the price of a status indicator with a manual magnetic reset facility will be multiplied in relation to the cost of an indicator with an indicating function only.
At present there are no design solutions on the market which enable rapid resetting of an optional but large number of indicators by means of a simple manual operation.
There is thus a great need for a status indicator which is superior to existing designs both from a functional, space-saving and cost point of view.